Scientists have just released details on their discovery of inter-galactic baryon material - "Dark Matter" that turns out to be regular matter, except that it is 'dark' - dispersed throughout the inter-galactic spaces and therefore not hot enough to detect. Sort of like it is all black-body radiation at very low temperatures.

Previously, it was assumed that inter-galactic space was empty, devoid of any usable resources.

My question is, 'By what means could this baryonic material be collected by a mega-huge multi-generational ship that has been sent on a one-way mission to spread humanity throughout the universe, in sufficient quantities to replenish supplies?'

Consider that the recent collision between two neutron stars, that produced the most recently-detected gravitational waves, probably created elements as dense as gold and platinum. Very useful elements to go after.

IT’S matter, but not as we know it. In July, an unexpected visitor
appeared at CERN’s Large Hadron Collider near Geneva, Switzerland.
Dubbed the pentaquark, this peculiar particle represents a
fundamentally new way to aggregate the basic building blocks of
matter. Although not forbidden by our current understanding of how
stuff comes together, it had never been conclusively spotted before.

He even argues that ordinary matter in extraordinary formations could
solve one of the greatest cosmological mysteries of our time – dark
matter.

Ideas about physics that are based on information and theories that are more than ten years old are, well, ancient history.

Contact with those aboard the ship would probably be lost by anyone in our galaxy, and they would be multi-generations before they arrived at their destination, but our 'seeds would be cast', randomly 'blowing in the inter-galactic wind'.

3 Answers
3

The intergalactic medium has 1 particle per cubic metre. According to wikipedia, 0.1% of these are heavy elements for the interstellar. Let's say that's true for the intergalactic for which I didn't find a value quickly. Let's be super optimistic and say all of them are useful and can be collected. You know what, let's make it even simpler. Let's assume all of them are iron. Now let's ask a very simple question: How much iron will be harvested on a journey from the milky way to the andromeda galaxy?

That's not the longest journey one can imagine, but it's pretty long. The distance is 2.5 million lightyears. That means 0.001 * 1/m³ * 2.365 * 10^22 m (that's your distance) = 2.365 * 10^19 / m². That's such a high number, I can't do much with it. So what is that in kg iron?

55.85 * 1.661 * 10**(−27) * 2.365 * 10^19 kg/m² = 2.19*10^(-6) kg/m².

That means in order for your spaceship to collect 1 kg iron in its journey, it needs a collector with a surface of about 455801 m². That means you need to build an iron collector that weighs less than 1 kg per 455801 m² surface area. Whatever the difference in weight between collector and collection rate and whatever you lose by having a giant, fragile apparatus out there in space is what you gain as net yield.

I don't know how big your ship is supposed to be, but I'd assume in almost any case it would be better to just bring the extra insert some heavy element before your journey starts. You can stop on some planet once you reach a new galaxy every million of years. Please be aware that you will not get only iron and of course a lot of elements you do not need (there is no way you'll end up with the ideal element distribution) and that you will manage to collect less in practice. A highly advanced, intergalactic civilization should also be able to just make whatever they please from hydrogen/helium, so there is little sense in collecting those special elements you are after.

Also consider: Since you need to build such a massive ship for collecting stuff being worth it, losing some atoms to the interstellar medium will hardly be noticed. The bigger your ship, the less loss per cubic metre (unless you are building a giant flat something), the less need for a collector.

But of course it is possible, see the ion trap. It's just not practical.

$\begingroup$The article you link to is about the interstellar medium, the question is about intergalactic medium. (I think, I could have misunderstood.)$\endgroup$
– Ville NiemiOct 27 '17 at 17:42

$\begingroup$You are right. But I'd guess it's about the same$\endgroup$
– Raditz_35Oct 27 '17 at 18:02

$\begingroup$The Wikipedia article has now become completely obsolete with this new discovery. Googling this will do no good. This is something that has not even been published. But it does not eliminate the relevance of the question. The details on the discovery are not necessary. If it makes it easier, assume the particles. But how do you harvest them? If google had an answer to this, I would not be asking the question.$\endgroup$
– Justin ThymeOct 27 '17 at 19:57

$\begingroup$.I am well aware the collector has to cover, not just cubic kilometers of space, but cubic megameters of space. Fragility is not an issue. A spider web is fragile,and doesn't take much material, but it still captures rather large insects. And this just has to collect atoms.$\endgroup$
– Justin ThymeOct 27 '17 at 20:06

I think that a huge multigenerational star ship would be unlikely to show any interest in attempting to collect interstellar gold and platinum. What would they do with them? In fact any such material would likely pose a significant hazard if present in any kind of particulate form due to the huge kinetic energy involved in an impact with a fast moving star ship.

That said other materials in interstellar space such as hydrogen might be useful. In the nineteen sixties Robert W. Bussard suggested that very tenuous clouds of interstellar hydrogen might be swept up by a vast electromagnetic scoop and used to fuel a fusion rocket. But the practicality of this so called Bussard Ramjet is debatable.

If you’re really set on trying to extract gold and platinum from interstellar space rather than from the much easier option of extracting it from a large metallic asteroid, then some form of modified Bussard ram jet might be of use.

Note it would not be possible to collect gold or platinum “baryon by baryon” in the same sense that you could not build an adult human by accumulating cells one at a time.

$\begingroup$They would use it for electrical systems and solid state devices. Every computer has gold in it. Highly conductive, extremely stable, and virtually completely non corrosive. Our technology would not function without it.$\endgroup$
– Justin ThymeOct 27 '17 at 19:03

1

$\begingroup$True gold can be a useful material; they would take as much as they needed, but no more. Given the situation why would they want to collect gold or platinum from the interstellar medium?$\endgroup$
– SlartyOct 27 '17 at 23:20

$\begingroup$Because in a trip lasting thousands of years, eventually the gold is no longer recoverable from high tech devices.$\endgroup$
– Justin ThymeOct 28 '17 at 3:34

$\begingroup$Ultimately however diffuse the gold is aboard ship it would always be far more concentrated than the gold in space and much easier to extract. But if the engineering problem is the need to find highly conductive and oxidation resistant materials for making microchips then there are a range of materials that could be used instead of gold. They might not be quite as good as gold but they would be adequate and they would be used because they would be so much more readily accessible. news.softpedia.com/news/…$\endgroup$
– SlartyOct 28 '17 at 7:25

$\begingroup$It would seem to me that you are just transferring the problem - eventually THOSE alloys will become scarce as well. But even recovering an ounce of gold in 100 years is STILL one ounce more that you didn't have. Obviously, in order to recover this intergalactic space stuff, the method would have to incorporate dome kind of 'concentrator'. Say, a funnel that brings it all to one spot.$\endgroup$
– Justin ThymeOct 28 '17 at 15:52

Dark matter has nothing to do with baryonic material aka conventional matter.

Anyways to answer your question,

Same ways all the previous space mineral harvesting go

mine the asteroids to get your materials

use bucket collectors to catch gas/dust as you pass through clouds

you could use a magnetic field to attract some particles

edit: I also wanted to point out that intergalactic matter would be incredibly sparse and most likely hydrogen

[hydrogen is the most abundant element generated from the BB, the heavier elements are believed to be generated mostly from stellar activities which happen 99% in galaxies so what would most likely be between galaxies is unused hydrogen]

$\begingroup$It's not dark matter, it is regular matter that is dark. The regular 'stuff' that is in the periodic table. Gold, platinum, iron, all the regular 'stuff'. But you would want to scoop it from an area of space much larger than the ship, to get enough. Say, several cubic megakilometers of space.$\endgroup$
– Justin ThymeOct 27 '17 at 15:59

$\begingroup$I get that, hence my answer, though your question at first glance seems to ask how to collect dark matter. Anyways, the dust is impractical to actively collect hence why I suggest passive options like the magnetic field and bucket collectors. In these areas there would still be asteroid fields which can be actively mined.$\endgroup$
– anonOct 27 '17 at 16:03

$\begingroup$Sorry for the confusion, my comment was not for you particularly, it was a generic comment that addressed your last line.$\endgroup$
– Justin ThymeOct 27 '17 at 16:10

$\begingroup$And mining the asteroid bit I have already considered and built in to the story. Whenever they come close (or even hit) an asteroid, there is a celebration of good fortune. The problem is in detecting asteroids in inter-galactic space and then getting to them while traveling at point 8 cee. You pretty much have to wait for one to hit you. The space ship is about ten km. across.$\endgroup$
– Justin ThymeOct 27 '17 at 16:32

$\begingroup$@JustinThyme That's what make intergalactic travel even harder than interstellar travel. Its akin to crossing a desert, you could waste energy trying to paw at the sand looking for water or you could use all your energy to get to the nearest oasis and then paw at the sand for water.$\endgroup$
– anonOct 27 '17 at 17:02